1. Field of the Invention
The present invention relates to a fabricating method of a carbon nanotube-based field effect transistor having an improved binding force with a substrate and a carbon nanotube-based field effect transistor fabricated by the fabricating method, and more particularly, to a fabricating method of a carbon nanotube-based field effect transistor having an improved binding force with a substrate by introducing an annealing process and a carbon nanotube-based field effect transistor fabricated by the fabricating method.
2. Discussion of Related Art
A carbon nanotube (CNT) is formed in a cylindrical structure in which six carbon atoms are connected to each other in a hexagonal shape. The carbon nanotube has a current density of about 1000 times higher than a copper wire and a carrier mobility of about 10 times higher than silicone, and thus has been widely used as a material of high sensitive/high speed electronic devices. Specifically, it can be applied to various fields such as a field emission device, a flat panel display, electrochemical field, energy storage field, etc.
In particular, when a carbon nanotube is used as a sensor, its sensitivity is very high at normal temperature. However, the carbon nanotube cannot be adsorbed uniformly and hysteresis occurs. In order to remove hysteresis of a carbon nanotube field effect transistor sensor, water molecules between the carbon nanotube and a substrate need to be removed. When there are water molecules, hysteresis occurs and a binding force between the carbon nanotube and the substrate decreases, which may reduce a lifespan of the sensor.
In order to solve such problems, an annealing process is introduced. FIG. 1 schematically shows a fabricating method of a carbon nanotube sensor introducing an annealing process. FIG. 2 shows voltage-current characteristics of a sensor fabricated by the fabricating method of FIG. 1.
Referring to FIG. 1, when a photoresist (PR)-patterned sample is dipped in a carbon nanotube (CNT) solution, all the CNTs are adsorbed onto a SiO2 surface and a photoresist (PR) surface. The photoresist (PR) surface has a lower hydrophilic property than that of the SiO2 surface, and thus fewer carbon nanotubes (CNTs) are adsorbed thereto. When the sample adsorbing the CNTs on its entire surface is immersed in an acetone solution to smelt the photoresist (PR) pattern, the carbon nanotubes (CNTs) adsorbed onto the photoresist (PR) surface are also removed. On the other hand, the carbon nanotubes (CNTs) adsorbed onto the SiO2 surface remain on the surface due to a van der Waals' force. Then, the sample from which the photoresist (PR) is removed is washed with methanol, isopropanol, and DI distilled water in sequence and then water on its surface is removed by high-purity nitrogen. During this process, the carbon nanotubes (CNTs) weakly or partially adsorbed onto the surface are desorbed from the surface. Therefore, it is not desirable to carry out an annealing process after the washing process to improve a binding force between a substrate and the carbon nanotubes (CNTs).
In order to solve this problem, an annealing process needs to be carried out right after the carbon nanotubes (CNTs) are adsorbed onto the SiO2 and photoresist (PR) surfaces. However, when the photoresist (PR) is annealed at 120° C. or more, polymers are cross-linked, and thus cannot be removed by acetone later. Therefore, a new process needs to be developed to solve such a problem.
Referring to FIG. 2(a), in case of a non-annealed sample, according to a change in source-drain current (ISD) with respect to a gate voltage (VG), (1) before detection, a p-type current-voltage characteristic is shown. (2) When the sample is washed with distilled water after first detection, more currents flow than before the detection. It can be understood that such a result is because a carbon nanotube bundle desorbed from another field effect transistor channel is adsorbed onto a field effect transistor channel under test during the washing process. (3) When the sample is washed with DI distilled water after second detection, a current does not flow. It can be understood that carbon nanotubes adsorbed onto the field effect transistor channel is desorbed during the washing process with the DI distilled water. In conclusion, in order to wash and repeatedly use a liquid sensor, a technology for improving a binding force between carbon nanotubes and a substrate is needed.
Referring to FIG. 2(b), in case of an annealed sample, according to a change in source-drain current (ISD) with respect to a gate voltage (VG), a p-type semiconductor characteristic is shown. (1) Before detection, (2) when the sample is washed with distilled water after first detection, and (3) when the sample is washed with distilled water after second detection, a p-type current-voltage characteristic is shown. However, it can be seen that as the number of washing processes is increased, a current level decreases. In conclusion, in order to wash and repeatedly use a liquid sensor, a technology for improving a binding force between carbon nanotubes and a substrate is needed.